1. Field of the Invention
The present invention relates generally to systems for growing diamond films, which employ electrodeless microwave reactor technology. More specifically, the present invention relates to systems, apparatus, and methods of employing electrodeless microwave plasma chemical vapor deposition (CVD) to produce high quality CVD diamond films.
2. Description of the Related Art
Chemical vapor deposition or CVD is a chemical process that can be used to produce various solid materials. In a typical plasma-enhanced CVD process, a substrate contained within a reactor chamber is exposed to a plasma discharge which activates one or more precursor gases and/or a carrier gas that reacts and/or decomposes on the substrate surface to produce the desired deposit. Plasma-enhanced CVD processes are often used to produce thin diamond films. CVD diamond has many outstanding properties, including hardness, stiffness, optical transparency, and high thermal conductivity. Because of the unusual process requirements for successful deposition of diamond films, e.g., the need for a high concentration of atomic hydrogen in order to stabilize the diamond structure over graphite and to drive the surface reactions involved in diamond deposition, diamond CVD reactors tend to be highly specialized. For example, microwave plasma reactors for diamond deposition tend to operate at higher gas pressures than many other plasma-based CVD processes and have much higher plasma power densities.
Various types of such apparatus (e.g., “reactors”) and methods exist for growing CVD diamond films from a vapor phase. Although many types of CVD techniques have been used for the growth of diamond films, the two primary techniques used for high quality thin film diamond are hot filament growth and plasma-enhanced growth. Typical plasma sources for plasma-enhanced CVD diamond growth are microwave discharge, RF thermal discharge, and DC arc discharge. The primary types of reactors employed include those that utilize electrodes to form plasma from gas, and those that do not One type of device that utilizes electrodes is described in U.S. Pat. No. 5,054,421 by Ito et al. titled “Substrate Cleaning Device.” The device can provide a relatively high rate of diamond deposition by employing an arc-jet plasma whereby gas is injected within a nozzle in which an arc is passing. The average gas temperature after passing the arc jet is significantly higher than can be achieved by thermal activation methods such as hot filament CVD. In an arc-jet flow, the velocity of gas flux is very high (˜1-10 km/sec), which results from the expansion or acceleration process. The high velocity of the gas flux allows for a more efficient delivery of the reaction species to a substrate due to a reduced boundary layer thicknesses, and thus, results in a higher linear growth rate. Despite the high linear growth rate, it is typically difficult to produce high quality diamond using arc-jet deposition, and any production, thereof, is spatially limited to a small coating area. Those two disadvantages restrict its ability to be utilized in applications involving direct use of the large, free-standing diamond plates, such as, for example, heat sinks or optical windows.
Although reactors that utilize electrodes have their purpose, when attempted to be employed to produce “transparent” diamond films, they typically result in excessive contamination and degradation, and thus, are generally considered in industry to be unsuitable for such purpose. To grow high quality diamond thin and thick films, electrodeless plasma CVD methods, in general, and microwave plasma CVD at 2.45 GHz and 915 MHz, in particular, have certainly the highest potential for large scale industrial application. Especially, if contamination-free films are required, radio frequency and microwave plasma CVD are generally considered the only suitable methods available, with the later being much more power efficient. Accordingly, the conventional technique to grow high quality and uniform diamond thin films, especially over large substrate areas, is through use of microwave plasma CVD methods employed using an electrodeless microwave CVD diamond reactor, such as, for example, that described in U.S. Pat. No. 5,175,019 by Purdes et al., title “Method for Depositing a Thin Film,” assigned to Texas Instruments Inc., Dallas Tex. An example of such device is provided by the ASTex PDS-19 CVD reactor marketed by Seki Technotron as Model No. AX6550.
Although key parameters to control diamond film quality and growth rate have strong interactions, such as, for example, microwave power density, substrate temperature, methane or other carbon precursor percentage, with respect to hydrogen (H2) flow (gas flow) and reaction chamber pressure, etc.; in conventional microwave CVD diamond reactors, the hydrogen flow rate is typically kept relative low, e.g., typically within a range of approximately 50-600 standard cubic centimeters per minute (sccm), or so, and with a very low velocity or flux. As such, gas flow is believed to have limited or no interaction with those key parameters. According to conventional wisdom, the main purpose of the low hydrogen flux is to not disturb the combustion chemistry inside the plasma discharge created by the microwave, otherwise, growth species depositions, such as hydrogen atoms and methyl radicals, would have an uneven distribution over the substrate surface area and result in significant non-uniform growth of the diamond film.
As manufacturers of conventional electrodeless reactors inject gas gently and with low flow rates of gas to prevent convection, often going to considerable design lengths to minimize the gas velocity at the substrate and the uniformity of gas flow, the maximum growth rates that may be achieved in producing optical grade CVD diamond using such conventional microwave plasma CVD apparatus and methods are only approximately one to three microns per hour over a large diameter substrate. As such, the deposition cost to generate a 4 in. diameter millimeter thick substrate can typically cost upwards of approximately $30,000.
The inventors, therefore, have recognized the need for an apparatus and method of employing electrodeless microwave plasma CVD technology to produce high quality CVD diamond at a substantially improved high growth rate, e.g., that approaching or exceeding that attainable through use of an arc jet, while maintaining high quality, e.g., superior transparency or high thermal conductivity, along with high film uniformity, and at a substantially reduced production cost.